The functioning principle of a class D amplifier uses a carrier signal of a relatively high frequency modulated by an input signal (Iin) to be amplified, of a frequency generally much smaller than the carrier frequency, and demodulates the amplified high frequency output signal. Generally, the frequency of the carrier signal is one or more orders of magnitude greater than the frequency of the modulated input signal.
An appropriate modulation for these applications is PWM, wherein the carrier signal being modulated is a square wave (Isquare or Isq) of a fixed frequency. The duty-cycle of the square wave is modified as a function of the input signal to be amplified. This is implemented by a triangular voltage waveform (+Vtri−Vtri) generated by injecting the square wave current signal into the virtual ground node of an integrating stage of the input signal (Iin) to be amplified, and which is then fed to an input of a successive fixed threshold comparator stage to generate the PWM modulated square wave signal that is amplified by a class D output stage, as shown in the circuit diagram of FIG. 1. Notably, the information content of the input signal may then be extracted from the amplified PWM modulated signal by way of a passive low-pass LC filter (called also a demodulation filter).